1. Field of the Invention
The present invention relates to light valve targets comprising dense arrays of electrostatically deflectable mirrors. More particularly, the invention pertains to targets for light valves comprising micro-mirrors arranged to be repelled from associated electrodes when electrostatically actuated by an electrical signal.
2. Background of the Prior Art
A cantilever beam deformable mirror target is a micromechanical array of deformable mirrors, each mounted as a cantilever beam, that can be electrostatically and individually deformed by an address means to modulate incident light in a linear or areal pattern. Used in conjunction with projection optics, a cantilever beam deformable mirror target can be employed for displays, optical information processing and electrophotographic printing. An early version employing metal cantilever beams fabricated on glass by vacuum evaporation is taught in U.S. patent Ser. No. 3,600,798. Another cantilever beam deformable mirrored device is described in an article by R. Thomas et al., "The Mirror-Matrix Tube: A Novel Light Valve For Projection Displays," ED-22 IEEE Tran. Elect. Dev. 765 (1975) and in U.S. patent Ser. Nos. 3,886,310 and 3,896,338. The device is fabricated by growing a thermal silicon dioxide layer on a silicon-on-sapphire substrate. The oxide is patterned in a cloverleaf array of four centrally-joined cantilever beams. The silicon is isotopically wet-etched until the oxide is undercut, leaving four oxide cantilever beams within each pixel supported by a central silicon support post. The cloverleaf array is then metallized with aluminum for reflectivity. The aluminum deposited on the sapphire substrate forms a reference grid electrode that is held at a d.c. bias. The device is addressed by a scanning electron beam that deposits a charge pattern on the cloverleaf beams, causing the beams to be deformed toward the reference grid by electrostatic attraction. Erasure is achieved by negatively biasing a closely-spaced external grid and flooding the device with low-energy electrons. A schlieren projector is used to convert the beam deflection caused by the deformation of the cantilevers into brightness variations at the projection screen. The cloverleaf geometry permits use of a simple cross-shaped schlieren stop to block out the fixed diffraction background signal without attenuation of the modulated diffraction signal.
Arrays of cantilever beam deformable mirrors integrated on silicon with addressing circuitry are also taught by the prior art. Such arrangements eliminate the electron beam addressing and its associated high voltage circuitry and vacuum envelopes. Devices of this type appear, for example, in the article of K. Petersen, "Micromechanical Light Modulator Array Fabricated on Silicon," 31 Appl. Phys. Lett. 521 (1977) and U.S. patent Ser. No. 4,229,732. The former reference describes a 16-by-1 array of diving board-shaped cantilever beams. The latter reference describes devices of different architecture, in which the cantilever beams are of square flap shape hinged at a corner. The flaps form a two-dimensional array rather than a one-dimensional row of diving boards with the wells beneath the flaps separated to permit addressing lines to be formed on the top surface of the silicon between the rows and columns of flaps. (The corner hinging of the flaps derives from the cloverleaf architecture taught by U.S. patent Ser. Nos. 3,886,310 and 3,896,338. The full cloverleaf architecture is not employed as this would preclude surface addressing lines since the cloverleaf flaps are hinged to a central post isolated from the silicon surface.)
A variation of the cantilever beam approach appears in the articles of K. Petersen, "Silicon Torsional Scanning Mirror," 24 IBM J. Res. Dev. 631 (1980) and M. Cadman et al., "New Micromechanical Display Using Thin Metallic Films," 4 IEEE Elec. Dev. Lett. 3 (1983). In this approach, metal flaps are formed that are connected to the surrounding reflective surface at two opposed corners. They operate by twisting the flaps along the axes formed by the connections. The flaps are not formed monolithicly with the underlying addressing substrate. Rather, they are glued to it in a manner analogous to deformable membrane devices.
The drawbacks of the cantilever beam spatial light modulators (SLM) described above include addressing circuitry that limits the fractional active area of the pixels, processing steps that result in low yields, sensitivity to film stress in the beam, beam insulator charging effects, lack of overvoltage protection against beam collapse, performance incompatibilities with low cost optics design and low contrast ratio due to non-planarized addressing circuitry on the surface. Such drawbacks are discussed in U.S. Pat. No. 5,172,262 of Hornbeck covering "Spatial Light Modulator and Method." That patent teaches an electrostatically deflectable beam spatial light modulator with the beam composed of two layers of aluminum alloy and the hinge connecting the beam to the remainder of the alloy formed in only one of the two layers, providing a thick, stiff beam and a thin compliant hinge. The alloy is on a spacer made of photoresist which, in turn, is on a semiconductor substrate. The substrate contains addressing circuitry in a preferred embodiment.
Each of the representative examples of prior art SLM's generally incorporates targets of dense arrays of reflective, electrostatically-deflectable mirrors. Each presents a mirror element structure characterized by a plurality of deflectable, reflective micro-mirrors. Each micro-mirror is spaced-apart from an underlying conductive or metallized electrode by means of an edge hinge or central post of insulative material such as positive photoresist. The insulative post or hinge lies within the only possible path of charge migration between mirror and electrode. As a consequence, potential differences are established between such micro-mirrors and the associated electrodes during operation. This occurs as quantities of charge are deposited upon selected mirrors (e.g. by a scanning electron beam modulated by a video signal input.) The resultant potential differences produce attractive forces that cause the deformable mirrors to deflect toward the associated electrodes.
The limitation of prior art SLM's to light valve targets that feature mirror elements operable exclusively in an attractive deflection mode imposes numerous problems. Many of such problems relate to inherent physical or structural limitations imposed by the size of the neutral air gap between the base electrode and the overlying micro-mirror.
A target comprising an array of micro-mirror elements is conventionally utilized in conjunction with a schlieren optical system. A beam of light is directed to the surface of the array (either directly or through a transparent substrate) which is conventionally addressed by means of a scanning electron beam. Predetermined amounts of charge are selectively deposited upon certain of the micro-mirrors. As mentioned above, the charged micro-mirrors are attracted to underlying base electrodes, each of which is tied to and therefore maintained at the potential of a surrounding grid. The uncharged micro-mirrors of the array remain undeflected and at a neutral attitude, their surfaces parallel to the underlying base electrodes. A 1/60 second scan, for the purpose of depositing charge upon predetermined mirrors of the array, occurs during each video frame while a charge "bleed-off" process takes place between the scanning of video frames.
The schlieren projection system can operate in one of two modes. In a first mode, light rays reflected from neutral micro-mirrors is returned to the optical source while those that are redirected upon reflection from deflected micro-mirrors are reconstituted and focused into a high-intensity image that is projected onto a screen. In an alternative mode, the schlieren optical system may be arranged to employ the light reflected from the neutral micro-mirrors to generate a high-intensity projected image while that reflected from the deflected micro-mirrors is discarded.
In either of the above modes of operation, it is well-known that the potential extent of micro-mirror deflection can significantly affect optical performance. For example, the larger the range of deflection, the larger the range of possible gray-level gradations. Targets formed of arrays of micro-mirrors operating in the attractive mode are strictly limited in this regard as each is subject to a maximum deflection angle that is a function of the height of the hinge joining the base electrode to the micro-mirror. While the maximum angle of deflection is a positive function of hinge height, this simultaneously establishes the size of the air gap between the neutral micro-mirror and its underlying base electrode.
Unfortunately, as hinge height is increased to increase maximum angle of deflection, negative consequences result from the corresponding increase in the size of the neutral air gap. In addition to fabrication problems which tend to limit the maximum height of the hinge, increased gap size increases operating voltage requirements. Such enhanced operating voltage requirements follow from the inverse-square relationship that defines the force fields for attracting the micro-mirrors. By increasing voltage requirements, large air gaps may make it impossible to employ alternative target addressing systems such as transistor matrices. Field effect transistors are typically scaled to tolerate 3.4 to 5 volts. Obviously, a device such as that taught by U.S. Pat. No. 5,172,262, which requires between seven and sixteen volts for micro-mirror deflection, cannot be integrated with a transistor matrix. Thus, the flexibility of application of an attractive-mode target is limited.
As mentioned earlier, attractive-mode devices encounter problems when employed in analog systems to produce gray-level images. It has been found that, when used in an analog deflection mode, an attractive-mode device can usefully employ only about one-third of the physically limited gap due to problems of electrostatic instability. When the attractive voltage reaches a level sufficient to deflect the micro-mirror by more than one-third of the gap, the electrostatic force tends to overwhelm the restoring spring force of the micro-mirror, causing it to snap all the way to the base electrode or close the gap. Physically, it is easy to see that, as the electrostatic attraction force is an inverse square function of separation distance, this force increases significantly as the gap is reduced.
The problem of gray-level operation has been addressed in prior art attractive-mode devices through their operation in a digital, rather than analog, mode. When operated digitally, the full gap height is employed with the micro-mirror allowed to "bottom out" or stop on the base electrode. Gray-level is established through time modulation, created by rapidly switching the micro-mirror between an open and a closed air gap with the proportion determining the shade of the projected image.
A number of problems are encountered when one employs such digital operation. The electronics for driving the light valve target, employing time division multiplexing, is much more complex than that required for analog. Further, the repeated opening and closure of the gap can produce a "sticking" effect due to Vanderwaals forces. Very careful preparation of the surface of the base electrode is required to address this problem, complicating the device and increasing production costs significantly.